1. Field of the Invention
The present invention generally relates to the conversion of chemical energy to electrical energy, and more particularly, to a nonaqueous electrolyte activating a rechargeable alkali metal electrochemical cell, particularly a lithium ion cell, designed for long cycle life and low temperature charge/discharge applications. These characteristics are provided, in part, by the activating electrolyte being a quaternary mixture of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate and ethylene carbonate with the former three carbonates being in their equilibrated molar mixture.
2. Prior Art
Japanese patent application disclosure No. H8-162153 (Toshiba Battery Co.) relates to a secondary lithium ion battery activated with a mixed solvent electrolyte of ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC) and ethylene carbonate (EC). The cell in this publication is stated to be capable of cycling at temperatures down to about xe2x88x9220xc2x0 C. However, this Japanese patent application disclosure makes no mention of providing the constituents of DMC:EMC:DEC in an equilibrated molar mixture. The problem is that as the cell is cycled between a discharged and a charged state, DMC and DEC undergo an ester exchange reaction to produce product EMC. If the ternary solvents of DMC:EMC:DEC are not filled into the cell in an equilibrated molar mixture, cell performance changes as DMC and DEC react to form EMC or EMC disproportionates to form DMC and DEC. In that light, a cell that is initially rated for discharge at xe2x88x9220xc2x0 C. may not be capable of such performance after several cycles as the percentages of DMC, DEC and EMC change. Such unpredictability is not acceptable in a lithium ion cell operating at such low temperatures.
According to the present invention, low temperature discharge and capacity fade problems are avoided by activating an alkali metal rechargeable cell, and particularly a lithium ion cell, with an equilibrated quaternary mixture of organic carbonate solvents in the electrolyte. The carbonate solvents and their mixture ratio are carefully selected to provide a lithium ion cell capable of discharge at temperatures below xe2x88x9220xc2x0 C. and that exhibits good cycling characteristics. At the same time, the cycleability of the present invention cells at room temperature is maintained as good as rechargeable cells of a similar electrode chemistry activated with conventional binary carbonate solvent electrolytes.
The object of the present invention is, therefore, to improve the low temperature charge/discharge performance of lithium ion cells by activating them with an electrolyte comprising an equilibrated quaternary mixture of nonaqueous organic carbonate solvents. A further objective of the present invention is to provide such electrolytes in lithium ion cells to improve their cycle life in comparison to cells of similar electrode chemistries activated with conventional binary carbonate and mixed carbonate/ester electrolytes.
For that purpose, the present invention is directed to an equilibrated quaternary mixed solvent system for an electrolyte activating a lithium ion cell, wherein the solvent system contains only organic carbonates. In particular, the preferred solvent mixture is EC:DMC:EMC:DEC, in which EC is in the range of about 20% to about 50%, DMC is in the range of about 12% to about 75%, EMC is in the range of about 5% to about 45% and DEC is in the range of about 3% to about 45%, by mole. This solvent system provides the cell with good low temperature discharge and cycling characteristics.
These and other objects of the present invention will become increasingly more apparent to those skilled in the art by reference to the following description and to the appended drawings.